1. Field of the Invention
This invention relates generally to a system and method for controlling the cathode airflow to a fuel cell stack in the event of a failure of the cathode input by-pass valve and, more particularly, to a system and method for controlling the airflow to the cathode side of a fuel cell stack in the event of a cathode input by-pass valve failure that includes maintaining a cathode exhaust gas valve in an open position, setting the speed of the stack compressor to open-loop control using a look-up table, measuring the airflow to the stack and limiting the current output from the stack based on the measured airflow.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. A hydrogen fuel cell is an electrochemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated in the anode to generate free hydrogen protons and electrons. The hydrogen protons pass through the electrolyte to the cathode. The hydrogen protons react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. The PEMFC generally includes a solid polymer electrolyte proton conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture and the membrane define a membrane electrode assembly (MEA). MEAs are relatively expensive to manufacture and require certain conditions for effective operation.
Several fuel cells are typically combined in a fuel cell stack to generate the desired power. For example, a typical fuel cell stack for a vehicle may have two hundred or more stacked fuel cells. The fuel cell stack receives a cathode input gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen input gas that flows into the anode side of the stack.
The fuel cell stack includes a series of bipolar plates positioned between the several MEAs in the stack, where the bipolar plates and the MEAs are positioned between two end plates. The bipolar plates include an anode side and a cathode side for adjacent fuel cells in the stack. Anode gas flow channels are provided on the anode side of the bipolar plates that allow the anode reactant gas to flow to the respective MEA. Cathode gas flow channels are provided on the cathode side of the bipolar plates that allow the cathode reactant gas to flow to the respective MEA. One end plate includes anode gas flow channels, and the other end plate includes cathode gas flow channels. The bipolar plates and end plates are made of a conductive material, such as stainless steel or a conductive composite. The end plates conduct the electricity generated by the fuel cells out of the stack.
The stack air compressor cannot operate at all flow and pressure combinations required by a fuel cell stack. Therefore, a fuel cell system will typically include a cathode by-pass valve that allows at least some of the airflow to by-pass the fuel cell stack and flow directly to the cathode exhaust. For example, there are points in the operating range of a fuel cell stack that require less cathode airflow than the compressor is capable of delivering at its minimum speed. During these conditions, the by-pass valve is used to redirect some of the compressor airflow to the exhaust.
In some fuel cell systems, the cathode by-pass valve defaults to the open position so that if the valve were to fail, much of the compressor airflow would be sent to the system exhaust. In response to the by-pass valve failure in the open position, the compressor will increase its speed until its maximum speed is reached in an attempt to deliver the airflow necessary to meet the stack power request. Further, with the by-pass valve in the open position, the system algorithms would attempt to increase the pressure in the fuel cell stack by closing the cathode exhaust gas valve. With the cathode exhaust gas valve closed to attempt to control the stack pressure, very little, if any, airflow will get to the stack because it will be flowing out the cathode exhaust through the by-pass valve.